An inkjet printing device known in the art prints images by ejecting ink droplets from a print head onto paper to form dots (print pixels). Printing is performed using a plurality of dot types, such as dark and light dots or large and small dots. To executed this type of printing, a technique for optimizing the proportions of dot types to improve image quality is required. For example, dot recording rates set for each dot type based on input gradation values may be compared to threshold values to generate dot data representing which dot is to be formed.